The Central Hypothesis of this renewal application is that under physiological conditions, prolactin (PRL) secretion is inhibited by dopamine (DA) and stimulated by oxytocin (OT) and that the activities of these neurons are regulated by higher hypothalamic input and PRL itself. We will test this hypothesis with a novel interdisciplinary experimental and mathematical approach to the following specific aims: (1) To determine the basis for a physiological interaction between DA, OT and PRL secretion, (2) To determine the physiological basis for the estrous cycle stage differences in the response of lactotropes to the PRL-releasing properties of OT, (3) To characterize the mechanism of action of OT on pituitary lactotropes and (4) To characterize the relationship of DA and other modulators of PRL release on OT-induced responses in lactotropes. These aims are significant because they will challenge accepted dogma that PRL secretion in physiological circumstances is only under inhibitory control of hypothalamic DA.